This invention relates to the use of nitrogen-free compounds as adhesion promoters for silicon-based scratch-resistant coatings on plastics.
Plastics are extremely versatile materials which have a number of desirable properties. However, a disadvantage of these materials is for example the susceptibility of their surfaces to physical damage. Various chemicals such as, for example, many solvents, may also cause considerable damage to the surfaces of plastics.
One method of protecting the surfaces of plastics from such damage is to apply a suitable coating. This, like the plastic itself, may in turn consist of a variety of materials. This is primarily dependent on whether the surface is to be protected rather from physical damage or from the action of chemicals. In the case of transparent plastics such as, for example, polycarbonates, superficial physical damage is of course particularly disadvantageous. Numerous coating materials which effectively protect in particular polycarbonates from physical damage are known. These are substantially silicon-based scratch-resistant coatings which are mostly condensation- or UV-curing. Examples may be found in J. Sol-Gel Sci. Techn. 1998, 11, 153-159, Abstr. 23rd Annual Conference in Organic Coatings, 1997, 271-279, EP-A 02 63 428, DE-A 29 14 427 and DE-A 43 38 361.
However, the problem frequently associated with the application of silicon-based scratch-resistant coatings is that the adhesion between plastic and coating is inadequate. There are already a number of known methods of nevertheless obtaining a satisfactory adhesion. These include physical methods such as, for example, plasma treatment or corona treatment, or the use of an adhesion promoter.
Many adhesion promoters react both with the surface of the plastic and with the scratch-resistant coating. (Covalent) chemical bonds may be formed in the process. In the case of the coating of polycarbonates, commonly used compounds include aminosilanes, such as aminopropyltrialkoxysilanes; the amino group reacts with the polycarbonate surface and the alkoxysilyl group reacts with the silicon-based scratch-resistant coating. However, nitrogen-containing adhesion promoters have the disadvantage that the basic nitrogen function (for example, a primary amine) causes considerably damage to the polycarbonate, a fact manifested inter alia visually by a considerable yellow coloration. A further serious disadvantage is that the adhesion of silicon-based scratch-resistant coatings rapidly diminishes during storage in water, in particular warm water, and the film ultimately peels off completely. In addition, bubble formation and clouding frequently occur.
Accordingly, the object of the present invention was the provision of adhesion promoters which lead to a good adhesion of silicon-based scratch-resistant coatings to polycarbonate, without having the above-mentioned disadvantages, such as detraction from the visual appearance (yellow coloration) and lability in water.
The present invention accordingly provides the use of nitrogen-free compounds corresponding to formula (I) and/or their hydrolysis products and condensation products
[R1(R2)j]iSiR3k(OR4)4xe2x88x92kxe2x88x92ixe2x80x83xe2x80x83(I),
wherein
i denotes the number 1, 2 or 3,
i denotes an integer from 1 to 10,
k denotes the number 0, 1 or 2,
k+i is xe2x89xa63, and
R1 denotes OH, OR, SH, SR, OOCxe2x80x94R, with R being C1-C10-alkyl or C6-C12-aryl,
R2 is an optionally singly or doubly substituted methylene group, wherein R1 is a preferred substituent, and the methylene groups in the chain may optionally be interrupted once or several times by hetero atoms,
R3 is an optionally substituted C1-C10-alkyl or C6-C12-aryl group and
R4 denotes a C1-C10-alkyl or C6-C12-aryl group, preferably a C1-C4-alkyl group,
as adhesion promoters for silicon-based scratch-resistant coatings on polycarbonate.